private void rotate(int angle)
        {
            int newSize = (int)Math.Sqrt(bm.Width * bm.Width + bm.Height * bm.Height);
            rotatedBm = new Bitmap(newSize, newSize);
            int x0 = bm.Width / 2;
            int y0 = bm.Height / 2;
            int difX = (int)(newSize / 2) - x0;
            int difY = (int)(newSize / 2) - y0;
            int newX, newY;
            double angleInRadian = (angle * Math.PI / 180);
            for (int x = 0; x < bm.Width; x++)
                for (int y = 0; y < bm.Height; y++)
                {
                    Color c = bm.GetPixel(x, y);
                    newX = (int) ((x-x0)*Math.Cos(angleInRadian) - (y - y0) *Math.Sin(angleInRadian) + x0) + difX;
                    newY = (int)((x-x0) * Math.Sin(angleInRadian) + (y - y0) *Math.Cos(angleInRadian) + y0) + difY;
                    rotatedBm.SetPixel(newX, newY, c);
                }
            pictureBox1.Image = rotatedBm;
            pictureBox1.Refresh();
        }